EDIT: Okay, I didn't explain paths: A path is a girl's romance side story with you and bad choices can cause you to lose paths. The UI indicates a lost path by greying out that character's portrait, like in your case. To troubleshoot where you went wrong, I recommend the walkthrough I linked below.
Here's a walkthrough, third download option, look for Luna's color (black).
You don't need any guide its a waste of time.
Every time you get a heart you did something right in a dialogue with a girl. If you didn't get a heart pulsating in the top corner of the screen just go backwards. Or go to a save before you picked one of the dialog options. Rinse and repeat you are now on every single girls path.
The game introduces this when you first install the game. If you didn't skip you would know this. This is a basic element that every single dating / jrpg / novel dating game like this has.
It's not that easy. A lot of decisions do work that way, yes: You pick an option and you get an affection point if you made the right choice. But there's also a bunch of choices that are more nouanced: Maybe you'll get the affection point later or maybe the game won't even show you the affection point gain. Maybe multiple choices yield affection points, but one yields more than the other.
Also, some people prefer to play the game in one go without having to rewind back and forth to experiment with different choices. That's what walkthrough mods are for. And for him it's already too late, he already lost a path. So I pointed him to a document that helps him understand what went wrong.
